method and device for cooling a motor

ABSTRACT

The invention relates to a method and to a device for cooling a motor, the motor driving at least one, at least two-stage compressor ( 2 ) of a coolant circuit ( 1 ), which includes at least one first compression stage and a second compression stage ( 4 ), and a coolant is conducted through the coolant circuit ( 1 ), which is brought from a low pressure level to a medium pressure level in the first compression stage ( 3 ), and from the medium pressure level to a high pressure level in the second compression stage ( 4 ), and which is then expanded to the medium pressure level following the second compression stage ( 4 ) while outputting heat.

The present invention relates to a method for cooling a motor according to the definition of the species of claim 1, and to a device for implementing the method.

Such a coolant circuit equipped with a two-stage compressor is used in connection with heat pumps, for instance. The two compression stages of the compressor are driven via a motor, to which it is connected for this purpose. With the aid of the first compression stage of the compressor, the coolant gas is compressed from a low level to a medium level. In the second compression stage, the pressure level is then increased further until the coolant has attained a high pressure level. Via a capacitor connected downstream from the second compression stage heat can then be output by the coolant, which is subsequently expanded and can once again absorb heat in an evaporator in order to be supplied in gaseous form to the first compression stage of the compressor.

Depending on the operating mode, for example, such a coolant circuit makes it possible to achieve heating or cooling of a space. The coolant is simultaneously used for cooling the motor, which is able to be operated at an optimal operating temperature in this way. In coolant circuits the motor of the compressor is usually cooled either by the coolant aspirated by the compressor or by the already compressed gaseous coolant. The waste heat of the motor is conveyed to the gaseous coolant either directly prior to or directly following the compression. In coolant circuits having two-stage compressors, which include a first compression stage and a second compression stage, it is known to cool the motor with the aid of the coolant that is at the medium pressure level. As a result, this will happen either by the aspirated gas prior to the second or second compression stage or by the pressurized gas of the first or first compression stage [sic]. Only gaseous coolant is normally used for cooling the motor in order to prevent a dilution of the lubricant oil provided for lubrication in the motor bearings inside the motor. This could lead to insufficient lubrication in the bearings and therefore cause damage to the motor. However, the cooling by aspirated gas prior to the compression in the second compression stage of the compressor reduces the efficiency of the coolant circuit because of the heating of the aspirated gas, in as much as it causes a decrease in the density of the aspirated gas and thus of the supplied mass flow. The cooling of the motor by pressurized gas after the compression in the first compression stage results in losses and increases the temperature of the pressurized gas of the first compression stage. This also increases the demands on the temperature stability of the motor.

To cool a motor of a two-stage compressor, it is known to divert from the main coolant flow of the coolant circuit a subflow flow after a capacitor connected downstream from the second compression stage, and to guide it to a motor cooling system via a type of bypass link. A separate expansion takes place in the bypass line, for which, depending on the operating mode, one or two expansion valves are required, which necessitate a separate control.

The known methods for cooling the motor therefore cause either a reduced efficiency of the overall system or they are relatively complex and able to be realized only by providing additional lines and expansion valves for which a separate control is required in addition.

The present invention is based on the objective of overcoming the disadvantages of the related art and, in particular, of providing a method and a device for cooling a motor in which the waste heat of the motor is returned into the cooling circuit without any negative effect on the efficiency of the overall system. The production expense and the control outlay should be as low as possible and the fewest components possible should suffice.

In the present invention, this objective is achieved by a method having the features of claim 1 and by a device having the features of claim 6. Advantageous developments are indicated in the dependent claims.

In a method for cooling at least one motor driving at least one compressor, having at least two stages, of a coolant circuit which includes at least one first compression stage and a second compression stage, and a coolant is conveyed through the coolant circuit, which is brought from a low pressure level to a medium pressure level in the first compression stage, and from a medium pressure level to a high pressure level in the second compression stage, and which is then expanded to the medium pressure level while dissipating heat following the second compression stage, the present invention provides for cooling of the motor by a two-phase coolant main flow that exhibits the medium pressure level.

The two-phase coolant main flow contains both gaseous and fluid coolant. Since the coolant main flow, that is to say, usually the entire coolant flow, is utilized for cooling the motor, no additional expansion valves are required. Accordingly, no corresponding control is necessary. The waste heat dissipated by the motor has no negative influence on the efficiency, since no negative effect arises either on the individual pressure side or the individual suction side of the compression stages.

In the case of coolant compressors that are provided with more than two compression stages, the method is used between two of the compression stages. In case of more than one coolant compressor, the method may be used between two coolant compressors. It is also possible to cool more than one motor, in which case each motor may possibly be allocated its own coolant circuit.

In one preferred further development, the coolant main flow is split into a fluid coolant component and a gaseous coolant component after cooling of the motor, the gaseous coolant component being supplied to the second compression stage and the fluid coolant component to the first compression stage of the two-stage compressor. After the motor has been cooled, the two-phase coolant main flow is therefore split up into the two phases, the gaseous component being compressed further in the second compression stage. In this way a very efficient compression of the gaseous coolant to a high pressure at a high temperature may take place directly in the second compression stage. A medium pressure accumulator, for example, may be used for separating the phases of the coolant from the coolant main flow, this being a collection container in which the coolant is separated into a gaseous component and a fluid component.

The coolant is preferably evaporated while absorbing heat before the first compression stage, and condensed while outputting heat following the second compression stage. Following the subdivision of the coolant main flow into the liquid and the gaseous coolant component, an expansion of the fluid coolant component therefore takes place, with a subsequent evaporation in an evaporator, in which heat from the environment is able to be absorbed, for example. This induces the previously fluid coolant component to transition to the gaseous phase as well, whereupon it is supplied, in gaseous form, to the first compression stage of the two-stage compressor, to be compressed and heated there. A capacitor may be provided, for instance following the second compression stage, in which the previously gaseous coolant is condensed and, for example, heat is output to the environment in the process. From there, the coolant is conducted further under high pressure and in partially liquid form and subsequently expanded to the medium pressure level.

In one preferred specific embodiments following the first compression stage, the coolant component is combined with the coolant component that has come from the second compression stage after emitting heat, before being used for cooling the motor. A combination of the two coolant components thus takes place, so that the entire coolant flow is available for cooling the motor.

In one alternative development, the coolant component is supplied directly to the second compression stage following the first compression stage; the center portion, which is gaseous once the motor has been cooled, is supplied to the second compression stage and combined with the coolant component coming from the first compression stage; the coolant that forms the coolant main flow and comes from the second compression stage after outputting heat is used for cooling the motor. This results in a relatively simple structure, and the entire coolant flow is used for cooling the motor even then.

In a device for implementing the aforementioned method, which is developed as a two-stage heat pump, climate control device or cooling system, in particular, it is provided according to the present invention that the device has a motor and a coolant circuit in which a two-stage compressor is situated, which has a first compression stage and a second compression stage able to be driven by the motor; and that a motor cooling system is integrated into the coolant circuit in such a way that a coolant main flow is able to be flow through it, and a phase separation element is situated downstream from the motor cooling system in the direction of flow, which is connected to the second compression stage via a suction gas line for a gaseous coolant component and to the first compression stage of the two-stage coolant compressor via a first line for a fluid coolant component.

This makes it possible to use the entire coolant that is at a medium pressure level for cooling the motor, and the waste heat to be recirculated into the coolant circuit. A negative effect on the efficiency therefore does not arise since a separation into a gaseous and a fluid coolant component takes place with the aid of the phase separation element once the waste heat of the motor has been absorbed or once the motor has been cooled, and only the gaseous coolant component is supplied to the second compression stage and condensed further thereby. It can therefore be operated with high efficiency.

An additional bypass link including additional expansion valves for supplying the motor cooling system can be dispensed with. Instead, the motor cooling system is simply traversed by the coolant main flow, which is able to absorb the corresponding heat. No additional control is required, for this purpose, so that the production and control expense is kept low.

In one preferred further refinement, an evaporator and possibly a throttle element as well additional components are disposed in the first line, upstream from the first compression stage. This makes it possible for the previously fluid coolant component to expand and to evaporate, so that it is able to be supplied to the first compression stage in gaseous form. Heat absorption from an environment, which is cooled as a result, takes place in the evaporator. The further components include filters or similar items, for example.

It is especially preferred that a capacitor and possibly a throttle element as well as possibly additional components are disposed in a second line of the coolant circuit, downstream from the second compression stage. Following the second compression stage, heat can be dissipated to the environment from the gaseous coolant component in the capacitor, which at least partially liquefies this coolant component. Because of the throttle element that follows, which may be developed as a simple throttle or as an expansion valve, for example, this coolant component is expanded, so that it is able to be utilized for cooling the motor at a medium pressure level in liquid and/or gaseous form. The further components, for example, may be developed as cooling elements for a power electronics system or a similar device.

In one preferred specific development, a mixing device, which is connected to a pressurized gas line coming from the first compression stage, and to the second line, is situated in the coolant circuit upstream from the motor cooling system. That is to say, the coolant component coming from the first compression stage and the coolant component coming from the second compression stage meet each other and can jointly be conducted from there to the motor cooling system. The entire coolant flow is therefore used for cooling the motor.

In one alternative development, the second line is connected to the motor cooling system, a pressurized gas line coming from the first compression stage discharging into the suction gas line leading to the second compression stage. The gaseous coolant component downstream from the phase separation element is able to be combined with the coolant component conveyed from the first to the second compression stage, upstream from the second compression stage. Even in this simplified design the entire coolant flow is routed to the motor cooling system where it is used for absorbing waste heat. However, the coolant component coming from the first compression stage is not combined with the coolant component coming from the second compression stage directly upstream from the motor cooling system, but first also travels through the second compression stage.

It is possible to place further components in a coolant line between the motor cooling system and the phase separation element. For example, these are throttle elements and/or additional cooling elements, which are used for cooling elements of a power electronics system, for instance.

The coolant compressor preferably has more than two compression stages, the method as recited in one of claims 1 through 5 being used between two of the compression stages. In this way even extensive cooling is achievable.

In one preferred further development, the device has two coolant compressors, and the method as recited in one of claims 1 through 5 is applied between the coolant compressors or between compression stages of the coolant compressors, it being possible to cool more than a single motor. In this way the device can be used in a very universal manner.

In the following text the present invention is described in greater detail based on preferred exemplary embodiments in conjunction with the drawing.

The figures show:

FIG. 1 a first specific development of a coolant circuit having a two-stage compressor; and

FIG. 2 a second specific development of a coolant circuit having a two-stage compressor.

FIG. 1 schematically shows a coolant circuit 1 of a heat pump, which has a two-stage compressor 2 equipped with a first compression stage 3 and a second compression stage 4. Two-stage compressor 2 is operated by a motor 5; a mechanical link between motor 5 and compression stages 3, 4 of two-stage compressor 2 is not shown for reasons of clarity.

With the aid of compression stages 3, 4 of two-stage compressor 2, the pressure level of a coolant is initially raised from a first pressure level to a medium pressure level, and then to a high pressure level. A liquid fluid under overpressure, which becomes gaseous following a pressure removal and the absorption of heat, is used as coolant. The coolant is conveyed in gaseous form, for instance, and at a low pressure to first compression stage 3 of compressor 2, where it is brought to a medium pressure level and heated at the same time.

Via a pressurized gas line 6, a gaseous coolant component in the coolant circuit according to the present invention then arrives at a mixing device 7 and is combined there with a coolant component that comes from second compression stage 4. This cooling component was supplied to the second compression stage of compressor 2 in gaseous form at a medium pressure level and was brought to a high pressure level in second compression stage 4 while being heated at the same time.

Following second compression stage 4, the gaseous coolant component is subsequently conveyed to a capacitor 9 via a second line 8. There, heat is output from the cooling component to an environment or a heat sink 10. The resulting condensed coolant component, which may include both liquid and gaseous phases, is subsequently expanded to the medium pressure level with the aid of a throttle element 11, which is developed as an expansion valve, for instance; coolant component arrives at mixing device 7 at this medium pressure and is combined with the coolant component coming from first compression stage 3.

The combined coolant components, i.e., the coolant main flow, which includes the entire volumetric flow, travels from mixing device 7 to a motor cooling system of motor 5 and absorbs heat from motor 5 there. Then, the coolant main flow is separated into the gaseous coolant component and the liquid cooling component in a phase separation element 12. The gaseous coolant component is subsequently conveyed to second compression stage 4 again.

The liquid cooling component is expanded with the aid of a throttle element 13, which may once again be developed as an expansion valve, and conveyed at a low pressure and low temperature to an evaporator 14 in which the liquid coolant component is transferred into a gaseous phase. In so doing, evaporator 14 absorbs heat from the environment or a heat sink 15, which is absorbed by the coolant component. Throttle element 13 and evaporator 14 are situated in a first line 16, which connects phase separation element 12 to first compression stage 3 of two-stage compressor 2. The pressure of the coolant component evaporated in evaporator 14 is then increased in first compression stage 3, so that it is able to be conveyed to mixing device 7 again at a medium pressure level and at an increased temperature.

FIG. 2 shows an alternative preferred exemplary embodiment, in which corresponding elements have been provided with matching reference numerals. In contrast to the exemplary embodiment according to FIG. 1, the coolant component is not conveyed to a mixing device upstream from the motor cooling system following first compression stage 3, but directly to second compression stage 4. Since the gaseous coolant component coming from phase separation element 12 is conveyed to second compression stage 4 as well, the coolant main flow is brought to the high pressure level in second compression stage 4 and heated in the process. Following the heat dissipation and condensation in condenser 9 and the subsequent expansion via throttle element 11, the coolant main flow, which has gaseous and liquid components, arrives at the cooling system of motor 5 and can the absorb heat there. The fluid coolant component separated from the coolant main flow by phase separation element 12 is conveyed via first line 16 and initially expanded to a low pressure level with the aid of throttle element 13. This is followed by an evaporation in evaporator 14, so that it is ultimately supplied to first compression stage 3 of compressor 3 in gaseous form, where it is brought to a medium pressure level while being heated at the same time, in order to then reach second compression stage 4.

In the method according to the present invention, or in the devices according to the present invention, which involve a heat pump system in particular, cooling of the motor that is required for driving the two-stage compressor takes place with the aid of the coolant main flow, i.e., by the entire coolant. An additional bypass link in order to divert a portion of the coolant for cooling the motor is not required. This results in a simplified design, especially on account of the reduced number of required expansion valves, and thus in a less complex control.

The waste heat from the motor is supplied to the coolant circuit again without reducing the efficiency of the overall system because of the phase separation that takes place after the waste heat has been absorbed.

With relatively little outlay, the procedure according to the present invention is adaptable to a coolant circuit having a single-stage compressor, an intermediate injection and an internal heat transmitter being able to be used or also an intermediate injection and a phase separation in a phase separation element.

Because of the design of the coolant circuit, a reversal of the cooling circuit for a defrosting and/or for the cooling operation is able to tabs place, although the flow must always traverse the phase separation element in the same direction. 

What is claimed is:
 1. A method for cooling a motor which drives at least one, at least two-stage compressor (2) of a coolant circuit (1), which includes at least one first compression stage (3) and a second compression stage (4), a coolant being routed through the coolant circuit (1), which is brought from a low pressure level to a medium pressure level in the first compression stage (3), and from the medium pressure level to a high pressure level in the second compression stage (4), and then is expanded to the medium pressure level following the second compression stage (4) while outputting heat, wherein the motor (5) is cooled by a two-phase coolant main flow, which has the medium pressure level.
 2. The method as recited in claim 1, wherein the coolant main flow is split into a liquid coolant component and a gaseous coolant component after cooling of the motor (5), the gaseous coolant component being supplied to the second compression stage (4), and the liquid fluid component being supplied to the first compression stage (3) of the two-stage compressor (2).
 3. The method as recited in claim 1 or 2, wherein the coolant is evaporated prior to the first compression stage (3) while absorbing heat, and is condensed following the second compression stage (4) while outputting heat.
 4. The method as recited in one of claims 1 through 3, wherein following the first compression stage (3), the coolant component is combined with the coolant component coming from the second compression stage (4) after outputting heat, prior to being utilized for cooling the motor (5).
 5. The method as recited in one of claims 1 through 3, wherein following the first compression stage (3), the coolant component is supplied directly to the second compression stage (4), the coolant component, which is gaseous after cooling of the motor (5), being combined with the coolant component coming from the first compression stage (3) and conveyed to the second compression stage (4), and the coolant that forms the coolant main flow and comes from the second compression stage (4) is used for cooling the motor (4) after outputting heat.
 6. A device, in particular a two-stage heat pump, climate-control device or cooling system, for implementing a method as recited in claims 1 through 5, wherein the device has a motor (5) and a coolant circuit (1), in which a two-stage compressor (2) having a first compression stage (3) and a second compression stage (4) is situated, which compressor is able to be driven by the motor (5), and a motor cooling system is integrated into the coolant circuit (1) in such a way that a coolant main flow is able to flow through it, a phase separation element (12) being disposed downstream from the motor cooling system in the direction of the flow, which is connected via a suction gas line (17) for a gaseous coolant component to the second compression stage (4), and via a first line (16) for a fluid coolant component to the first compression stage (3) of the two-stage coolant compressor (2).
 7. The device as recited in claim 6, wherein an evaporator (14) and possibly a throttle element (13) and possibly further components are situated in the first line (16) upstream from the first compression stage (3).
 8. The device as recited in claim 6 or 7, wherein a capacitor (9) and possibly a throttle element (11) as well as possibly further components are situated in a second line (8) of the coolant circuit (1) downstream from the second compression stage (4).
 9. The device as recited in one of claims 6 through 8, wherein a mixing device is situated in the coolant circuit (1) upstream from the motor cooling system, which is connected to a pressurized gas line (6) coming from the first compression stage (3), and to the second line (8).
 10. The device as recited in one of claims 6 through 8, wherein the second line (8) is connected to the motor cooling system, and a pressurized gas line (6) coming from the first compression stage (3) discharges into the suction gas line (17) leading to the second compression stage (4).
 11. The device as recited in one of claims 6 through 8, wherein additional components are situated in a coolant line between the motor cooling system (5) and the phase separation element (12).
 12. The device as recited in one of claims 6 through 11, wherein the coolant compressor (2) has more than two compression stages (3, 4), and the method as recited in one of claims 1 through 5 is used between two of the compression stages.
 13. The device as recited in one of claims 6 through 11, wherein it has at least two coolant compressors, and the method as recited in one of claims 1 through 5 is used between the coolant compressors or between compression stages of the coolant compressor, possibly more than one motor being cooled. 